/Engine/Source/Runtime/Engine/Public/Subsystems/EngineSubsystem.h
UEngineSubsystems are dynamic and will be initialized when the module is loaded if necessary. This means that after StartupModule() is called on the module containing a subsystem, the subsystem collection with instantiate and initialize the subsystem automatically. If the subsystem collection is created post module load then the instances will be created at collection initialization time.
class UEngineSubsystem : public [UDynamicSubsystem](API\Runtime\Engine\Subsystems\UDynamicSubsystem)